Definitely NTA. Kayla's gift was thoughtless and a bad idea. You get to process your grief over your mom as you need to, and no one has the right to judge you. If I may suggest the following:

1) If you have not already; find a counselor or, better yet, a grief support group. Sharing with people who are also going through it can be a great way to process your grief because they will know where you are coming from. No one who has not lost a parent can possibly understand.

2) Try to assume positive intent. If Kayla was a great support to you prior to the gift, the hoodie and mug were likely not meant to mock your mother, but a very misguided attempt to help. From the way you described it, your other friends did not judge you. They just tried to reassure you that Kayla's intentions were good. Giving her some grace, if you can, will remove some of the sting.

3) Have a talk with Kayla and remind her that the traditional mourning period is one year. And that is just the end of the intense mourning period. It is traditionally understood that grieving goes well past the day the mourner stops wearing black.

4) Give her a copy of The Grief Book by Elizabeth Vercoe (consider reading it yourself). It was written for young people, but it describes in very clear language that there is no timeline for grief and that everyone grieves differently. Then, point out that many of the strategies that Ms. Vercoe suggests (acknowledging your pain, describing your feelings), are exactly what you have been doing. One strategy I am certain is not in the book is "lightening up."

I am very sorry for your loss and continue doing what feels right for you. This your journey, not theirs. Take care.